1. Field of the Invention
The present invention relates to a fuel cell system and a scavenging method for use in a fuel cell system, in which scavenging of at least one of a fuel gas flow field and an oxygen-containing gas flow field can be performed at the time of stopping power generation, or after stopping power generation, in order to prepare for the initiation of a next operation of the fuel cell system, which operates at a low temperature, such as a temperature below the freezing point.
2. Description of the Related Art
By way of example, a polymer electrolyte fuel cell employs a membrane electrode assembly, which includes an anode (fuel electrode) and a cathode (air electrode), and a polymer electrolyte membrane interposed between the electrodes. The electrolyte membrane is an ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. A fuel gas flow field is formed between the anode and one of the separators, and an oxygen-containing gas flow field is formed between the cathode and the other separator. In use, normally, a predetermined number of membrane electrode assemblies together with separators are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas flow field. The fuel gas flows through the fuel gas flow field along the anode. A catalyst within the anode induces a chemical reaction with the fuel gas, in order to split hydrogen molecules into hydrogen ions and electrons. The hydrogen ions move toward the cathode through a suitably humidified electrolyte membrane, and the electrons flow through an external circuit to the cathode, thus creating DC electrical energy. Further, in the fuel cell, an oxygen-containing gas such as air is supplied to the oxygen-containing gas flow field, and the oxygen-containing gas flows along the cathode for causing a reaction. At the cathode, hydrogen ions from the anode combine with electrons and oxygen, producing water. Water is also retained at the anode due to back diffusion from the cathode, or as a result of high humidification of the fuel gas.
If water at any of the electrodes becomes excessive, water clogging may occur. Thus, in the fuel cell system of this type, when stopping operation of the fuel cell system, in order to achieve a desired performance when initiating the next operation of the fuel cell system, a technique for scavenging both sides of the anode and the cathode has been proposed. In such an anode/cathode scavenging technique, the oxygen-containing gas is supplied to the anode as well as to the cathode in order, for example, to remove water produced during power generation from the membrane electrode assembly or from the separators in the fuel cell (see Japanese Laid-Open Patent Publication No. 2003-331893).
In this anode/cathode scavenging technique, in order to remove liquid droplets, the oxygen-containing gas is supplied at a large flow rate. Thus, while performing the scavenging process, an air compressor for discharging oxygen-containing gas, such as air, is operated with high power.
However, in the fuel cell system, if the power generation period from start to end is short, in some cases, the amount of water produced by power generation is small at the anode, and large only at the cathode. Even in such cases, in the above anode/cathode scavenging process, which is performed at a large flow rate, a large capacity air compressor is required. Further, when the air compressor is operated at a large flow rate, loud noises are produced disadvantageously. Further, the high capacity air compressor is large and heavy, and consumes a large amount of energy compared to normal capacity air compressors.
Further, after the fuel cell system has stopped, when the outside temperature thereof decreases and the fuel cell is started at a low temperature, such as a temperature below the freezing point, before the fuel cell has fully warmed up, the ignition switch may be turned off by an operator such as a driver. Therefore, if operation of the fuel cell system is stopped after operation of the fuel cell system has been started at a temperature below the freezing point, and power generation is performed for a short period of time, or stated otherwise, if operation of the fuel cell system is stopped by action of the driver in a short period of time after operation of the fuel cell has been started at a temperature below the freezing point, it has been found, in some cases, that the fuel cell system becomes unstable due to insufficient activity of the electrolyte membrane.
A technique for eliminating such instability beforehand, in order to achieve desired performance when initiating a next operation of the fuel cell system, has been proposed (see Japanese Laid-Open Patent Publication No. 2004-152599).
According to this technique, immediately after operation of the fuel cell system has been started at a temperature below the freezing point, if the ignition switch is turned off, halting power generation by the fuel cell is prohibited until the temperature of the fuel cell reaches a predetermined value or more. Thus, the electrolyte membrane is always kept sufficiently active, and stable performance when initiating a next operation can be achieved.
However, in the technique disclosed in Japanese Laid-Open Patent Publication 2004-152599, immediately after operation of the fuel cell system has been started at a temperature below the freezing point, if the ignition switch is turned off, halting of power generation by the fuel cell is prohibited until the temperature of the fuel cell reaches or exceeds a predetermined value. In this case, the operator of a moving object, such as the fuel cell vehicle, may feel a sense of discomfort, since the fuel cell system is operated to perform power generation even though the ignition switch has been turned off.
Further, in the technique disclosed in Japanese Laid-Open Patent Publication No. 2003-331893, when water on both the anode and cathode sides is discharged simultaneously upon stopping the fuel cell system, a large capacity air compressor, which produces a large amount of wind, is required. Therefore, energy consumption is large, and large noises are produced.